The spinal cord and nerve roots, or neural elements, pass through the spinal canal, which is composed of vertebrae and ligaments that circumferentially surround the neural elements. When the spinal canal is narrowed, the neural elements can be compressed. The most common condition in which the spinal canal is narrowed is spinal stenosis, which is generally the result of multifactorial degenerative processes usually related to advancing age or repetitive movements of the spine that lead to “wear and tear.” In this condition, normal anatomic elements of the spine (discs, ligaments, facets, vertebral endplates) can hypertrophy and degenerate, with a net result of a narrower spinal canal and compression of the neural elements. This compression can cause limb weakness, numbness, pain and gait disturbance.
Spinal decompression can be surgically performed to alleviate the compression of the neural elements with the goal of halting or improving limb weakness, numbness, pain and gait disturbance. One such technique for spinal decompression is spinal laminoplasty, which results in expansion of the spinal canal, thereby alleviating compression of the neural elements. Spinal laminoplasty is performed from the back, or posterior aspect, of the spine. The laminae are the bony elements of the spinal canal posteriorly which are cut during this procedure in order to result in expansion of the spinal canal.
In performing an open door laminoplasty, a drill or other bone cutting device is used to create a full thickness cut through the bony ring surrounding the neural elements at the junction of the laminae and facets on one side; on the other side, a drill or other bone cutting device is usually used to create a groove between the laminae and facets. The unattached laminae from the side with the full thickness cut are then wedged away from the spinal canal and the ligament underneath the laminae is removed. This decompresses the spinal canal by removing a constrictive element and expanding the size of the spinal canal. In a French door laminoplasty, a full thickness cut is made through the midline spinous processes and the midline laminae, enabling wedging open of the spinal canal in the midline as opposed to one side, which is the case with open-door laminoplasty.
An implant is then inserted between laminae and facets or between the two aspects of the spinous process. This implant acts as a connecting bridge between the two ends of cut bone and maintains the expansion of the spinal canal, similar to adding an extra leaf to a dining room table in order to expand it. The implant helps maintain expansion of the spinal canal and thereby decompression of the neural elements because the cross-sectional area of the spinal canal is larger, post-surgery. The implant can be made of cadaver bone, patient's bone, synthetics, metal alloy or a combination thereof.
Many variables in combination determine the best sized implant that should be utilized for laminoplasty, including the patient's anatomy, extent and location of spinal stenosis, and the width of the full thickness cut created during laminoplasty. The width of the cut created during laminoplasty is variable and cannot be predicted ahead of time. In this context, current implants for laminoplasty which are fixed in size have limitations in their versatility because they are not adjustable and can be cumbersome to switch out in favor of a possibly better sized implant if the originally placed implant is the wrong size. Even if the surgeon measures the size of the gap created between two ends of cut bone at the intended site for the laminoplasty implant and either tries to fashion an appropriately sized implant from cadaveric or patient bone or tries to select and modify a prefabricated one from a manufacturer, this can be time consuming and still result in an implant that does not perfectly fit the size and shape of the gap created by the laminoplasty. The gap created when the bone is cut full thickness at the intended implant site is not adjustable once it has been created. A fixed size implant limits the options available for expanding the spinal canal. Its fixed size may be too large or too small and unable to be adjusted to fit the specific anatomic needs of each individual patient. An implant that can be adjusted in situ would allow expansion of the spinal canal at the appropriate length specific to the patient's particular anatomy. None of the currently available implant options are adjustable or customizable in situ in order to appropriately fit the size of the gap whose dimensions itself are unpredictable before surgery and not modifiable once created. This fact limits the range of surgically acceptable configurations that can be achieved through the use of currently available implants. In addition, current implants do not result in any further expansion of the spinal canal should this be required once they are implanted because they are of fixed dimension.
There is therefore a need in the art for an improved laminoplasty device that enables length adjustment while and after it is implanted and also distracts or increases the size of the surgically created gap. An implant that can expand or retract after it has been placed in the laminoplasty gap to meet certain anatomic constraints allows its length to be varied and cancels the need for a custom shaped implant that is time consuming to create and can still be poorly fitted.